For transactions concerning physical goods, there are typically four parties involved: the seller, the buyer, the payment system, and the delivery mechanism. While physical goods could be exchanged locally without a formal delivery system, the tracking information provided by the delivery system is invaluable for resolving disputes between the buyer and the seller.
With digital goods, the seller could theoretically send the digital good directly to the buyer, either through e-mail, an online server, or in a closed-loop environment. One of the problems with such an arrangement is that there is no objective third party tracking of the consummation of the transaction. Without this information, the buyer is vulnerable to non-delivery, and the seller is vulnerable to fraudulent chargebacks.